Means for shielding electron guns of cathode ray tubes against spurious charges



ELLE-+149 r i Apnl 2, 1968 J. w. SCHWARTZ 3,376,443

'MEANS FOR SHIELDING ELECTRON GUNS OF CATHODE RAY TUBES AGAINST SPURIOUSCHARGES Filed Jan. 14, 1966 Ill/I FIG. I

INVENTOR: JAMES W. SCHWARTZ BY Wmfid ATT' Y5 United States Patent Office3,376,448 Patented Apr. 2, 1968 3,376,448 MEANS FOR SHIELDING ELECTRONGUNS F CATHODE RAY TUBES AGAINST SPURIOUS CHARGES James W. Schwartz,Western Springs, 11]., assignor to National Video Corporation, Chicago,Ill., a corporation of Illinois Filed Jan. 14, 1966, Ser. No. 520,780 2Claims. (Cl. 313-82) ABSTRACT OF THE DISCLOSURE This invention relatesto an electron gun for cathode ray tubes, and, more particularly, to anelectron gun developing a plurality of beams such as is useful in colortelevision.

The problem solved by the instant invention is that of improper beamlanding or divergence from desired patterns due to the adverse effect ofspurious electrostatic charges about the electron gun, particularly onthe glass envelope and other gun-supporting structure. This isparticularly important in the areas of the exposed or unshieldedelectron beam such as the gap between the focusing lens cylinders G-3and G4. In black and white television picture tubes and other types ofkinescopes, the beam is shielded by the construction of the overlappingfocusing lens. Such lenses in color television are not feasible becauseof size and operational problems.

In color television tubes, for example, the spurious charges manifesttheir presence in the form of improper lines about the images presentedon the face of the cathode ray tube. An image may be outlined in one ofthe colors because of improper convergence. In most instances, thespurious charges are transient so that any compensating adjustment ofthe convergence fails to permanently remedy the defect of convergencedrift. Arcing between gun elements also changes spurious distributionand adversely affects convergence. The convergence drift is particularlyvexing in the case of color television sets where the convergenceadjustment procedure is quite complicated and not easily performed inthe home.

The expedients employed for remedying this problem have not beensuccessful. Although it is appreciated that the spurious charges may besubstantially eliminated by specialized manufacturing technique, i.e.,use of special glass, careful cleaning, special surface treatment, etc.,such are not compatible with the high volume, low cost production oftubes. Further, the use of shields coaxial with the gun and neck of thetube have been unreliable because of the tendency of arcing to occur.

It is the general object of this invention to avoid the difiicultyoutlined above and more specifically to provide a novel means and methodwhereby the adverse effects on the electron beam by virtue of spuriouscharges about the electron gun are substantially minimized. Anotherobject is to provide a solution to this problem which is compatible withlow cost-high volume production of cathode ray tubes. Other objects andadvantages of the invention may be seen in the details of constructionand operation set down in this specification.

The invention is explained in conjunction with an illustrativeembodiment in the accompanying drawing, in which:

FIG. 1 is a fragmentary elevational view, partially in section, of thegun-end of a cathode ray tube such as is employed in color television;

FIG. 2 is a perspective view of the grid portion of the electron gun ofFIG. 1;

FIGS. 3 and 4 are enlarged sectional views through the axis of one ofthe series of coaxial cylinder gun portions of FIG. 1 with the FIG. 3showing lacking the inventive shielding of FIGS. 1 and 4; and

FIG. 5 is an enlarged perspective view of a shield element employed inthe apparatus of FIGS. 1, 2 and 4.

In the illustration given and with particular reference to FIG. 1, thenumeral 10 designates generally the neck of the glass envelope or tubeused in color television and the like. A stem assembly generallydesignated 11 is provided at one end of the tube and sealed to the glassof the tube in conventional fashion. The stem assembly is connected byleads 12 to the three electron guns 13, 14 and 15 (see FIG. 2). Theassembly 15 is shown in cross-section in FIG. 1 and, like the otherassemblies 13 and 14, includes four grids 16-19 in proceeding forwardly(in the direction of the electron beam) from the stem 11. Inconventional parlance, the first grid is usually designated G-1 and isreferred to herein by the numeral 16. The second or acceleration grid6-2 is designated 17. The focusing grid G-3 is designated by the numeral18, and the second acceleration grid or anode (3-4 is designated by thenumeral 19. The gun assembly includes a convergence assembly generallydesignated 20 coupled to the three guns 13-15.

Reference to FIGS. 3 and 4 reveals that the G-3 grid 18 defines with theG-4 grid 19 a gap (see reference numeral 21 in FIG. 1). At the edges ofthe grids 18 and 19 defining the gap 21, the grids 18 and 19 areequipped with rolled edges as at 22 and 23, respectively. Thissubstantially eliminates the tendency of arcing to take place across thegap 21 which might occur if the edges were sharp. Also seen in FIGS. 3and 4 is a schematic representation of the electron beam designated 24and 24', respectively, which is originated at the cathode 15 anddeveloped by the lines of force created by the difference in potentialbetween the grids .16, :17, 18 and 19. For example, the grid 19 may beat 15,000 volts, while the grid 18 may be at 5500 volts. Theequipotential field lines resulting from this potential differential andgrid geometry assume the usual lens-like configuration, resulting in theformation of the electron beam 24.

Still referring to FIGS. 3 and 4, it Will be seen that a spurious chargeschematically represented by an electron and designated 25 islocatedadjacent the glass envelope 10 and radially outwardly of the gap 21.Such a charge 25 develops an electrostatic force pattern which extendsthrough the gap 21 and adversely affects the electron beam 24. In manyinstances, the electron beam 24 will be displaced from its intended pathor trajectory (compare FIGS. 3 and 4), the proper path (as at 24' inFIG. 4) being that established under conditions wherein spurious chargesare substantially absent. In still other cases, the beam may bedistorted from the desirably round crosssection into a cross-sectionalpattern.

In still other instances, the spurious charge may be present on theinsulating rods 26 which constitute part of the gun-supporting assemblygenerally designated 27. The gun assembly includes a centerbrace bracket28 interconnecting the three guns 13-15 and a plurality of supportingstraps 29 interconnecting the three guns with the insulating rods 26 (asshown) and the various grids 16-19. The insulating rods 26 areconventionally constructed of glass material such as porous Pyrex 10.

I find that the adverse effect on the electron beam 24 of spuriouscharges (such as that represented at in FIGS. 3 and 4) can besubstantially minimized through the use of shield means such as thatdesignated in FIGS. 4 and 5. The shield means includes a shield elementof segmental circular construction which is welded as at 31 to itsassociated grid. Optimally, the shield elements 3t) extent radiallyoutwardly of the associated grid toward but short of the envelope 10,abutting the rolled or curled edges 22 and 23 of the grids 18 and 19.Through the use of shield elements extending laterally outwardly towardthe most deleterious location of the spurious charge, the lines of forceordinarily emanating from the spurious charge directed through the gap21 are substantially eliminated or markedly reduced.

In one specific example of the invention, the G-3 and G4 grids 18 and19, respectively, were of the equal diameter cylinder type having insidediameters of 0.353 with the adjacent circular edges rolled to form thecurls 22 and 23 on radii that develop outside diameters of 0.440". Withthe guns so constructed and without the shield elements 30 installed,arcing was induced, using a high frequency Tesla coil circumscribing thegaps 21, the Tesla coil being operated at kv. Checking out the beamlandings before and after the arcing induced by the Tesla coil revealeda shaft of the green, red and blue dots of the order of 0020-0050".Results of a larger magnitude in beam shift were observed when theenvelope 10 was wrapped with foil circumscribing the gaps 21 and chargedto 25 kv., thus simulating the effects of spurious charges. When theshield elements were installed, the same tests produced no significantchange in landing position. In the specific illustration given, eachshield element (a total of six being employed) extended over a 120sector between the various multi-forms and were constructed of 0.031"hydrogen-fired non-magnetic stainless steel. Each shield element 30 hada rounded outer edge and rounded corners at the areas adjacent themultiform.

It is to be appreciated that in the practice of the invention, the size,shape and location of the shield elements 30 are interrelated.Optimally, the shield elements 30 should be no closer than the adjacentparts of the curled edge portions 22 and 23; otherwise, the tendency toare may be increased. The closer the shields 30 are positioned relativeto each other, the less they need to extend radially outwardly of thegap 21 to achieve the same minimization of adverse effect on theelectron beam 24. Thus, in some instances, it may be advantageous toprovide the shields 30 as integral portions of the grids as by extendinglaterally outwardly the curled portions 22 and 23. In the ordinarymanufacturing operation, I find that most advantageously the shieldelements 30 are installed as illustrated. Further, in some instances itmay be advantageous to depart from the planar configuration of shieldelement illustrated, introducing a divergence as the one proceedsradially outwardly from the electron beam 24. I prefer for the area ofthe shield element adjacent the associated grid to be essentiallyplanar, but this can be satisfactorily approximated in certain instancesby using elements of slight curvature, as in having a paraboliccurvature in longitudinal section.

Generally speaking, the further the shield elements are spacedlongitudinally from the gap 21, the greater their size must be, i.e.,radial extent, of the shield elements 30, so as to minimize thepossibility of penetration of lines of force from any spurious charge inthe supporting or enveloping structure associated with the electron gun.

In the illustrated example, by positioning the shield elements 30immediately adjacent the curled portions 22 and 23, I achieve a spacingshield elements 30 of the order of the width of the gap plus 0.070".Tests performed with the spacing of the shield elements enlarged to thegap plus 0.150" produced significantly poorer results in controlling theconvergence of the electron beams 24. Further, I prefer to terminate theperipheral edges 30a of the shield elements a spaced distance from theenvelope 10, so as to minimize any possibility that the tube 10 mightcrack during the heat treatment due to inadvertent contact between theshield element and the tube because of the different coeflicients ofthermal expansion of the materials employed in these two structures.

While, in the foregoing specification, a detailed description of anembodiments of the invention has been set down for the purpose ofillustration, many variations in the details herein given may be made bythose skilled in the art without departing from the spirit and scope ofthe invention.

I claim:

1. In a cathode ray tube having a plurality of guns for generatingseparate and parallel electron beams mounted in a constricted neckportion of the tube envelope, each gun including a plurality ofcylindrical grids spaced along a common axis and electrically isolatedfrom one another, two adjacent of said grids in each gun being afocusing grid and an accelerating grid operated at a potentialdifference of about 20,000 volts and defining opposing rolled edges forpreventing arcing, the improvement comprising: a shield element for eachgun adjacent the rolled edge of said focusing grid and extendingradially outward beyond said rolled edge toward said constricted neckportion of said envelope, and a corresponding shield element for eachgun adjacent the rolled edge of said accelerating grid and extendingradially outward beyond said rolled edge toward said constricted neckportion of said envelope, said shield elements associated with each guncooperating to minimize the effect of spurious charges in said tube.

2. The structure of claim 1 wherein there are three such guns defining atransverse triangle and each of said shield elements is a fiat, circularsegment attached to its associated grid.

References Cited UNITED STATES PATENTS 2,971,118 2/1961 Burdick 3l382 XJAMES W. LAWRENCE, Primary Examiner. V. LAFRANCI-II, Assistant Examiner.

